Continuing innovations in the field of fiber optic technology have contributed to the increasing use of optical fibers in telecommunications and data communications networks. With the increased utilization of optical fibers, there is a need for efficient peripheral devices that assist in the transmission of data through these optical fibers, such as optical switches. An optical switch operates to selectively couple an optical fiber to one of two or more alternative optical fibers such that the two coupled optical fibers are in communication with each other.
The coupling of the optical fibers performed by an optical switch can be effectuated through various techniques. One technique of interest utilizes micro-mirrors to selectively route optical signals from an input optical fiber to a selected output optical fiber. In the simplest implementation of the micro-mirror technique, the input optical fiber is aligned with one of two output optical fibers, such that when the micro-mirror is not placed in the optical path between these two aligned optical fibers, the two aligned optical fibers are in a communicating state. However, when the micro-mirror is interposed between the two aligned optical fibers, the micro-mirror steers, i.e., reflects, the optical signals from the input optical fiber to the other output optical fiber. The positioning of the micro-mirror in and out of the optical path between the two aligned optical fibers can be accomplished by using a micro-machined actuator that mechanically displaces the micro-mirror to a desired position.
Another technique of interest utilizes thermally created bubbles, instead of micro-mirrors, to selectively route optical signals from input fibers to target output optical fibers. This technique is implemented in a thermally activated optical switch that is described in U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention. A conventional thermally activated optical switch 10 is schematically illustrated in FIGS. 1 and 2. As shown in FIG. 1, the optical switch includes a waveguide chip 12, a heater chip 14 , and a metal substrate 16. The waveguide chip contains planar waveguides 18, shown in FIG. 2, that serve as media for transmission of optical signals. These waveguides form a matrix of optical paths. Optical paths 20, 22, 24 and 26 facilitate lateral transmissions of optical signals, while optical paths 28, 30, 32 and 34 facilitate vertical transmissions of optical signals. The waveguide chip also contains a number of trenches 36, located at intersections of optical paths. Each trench is positioned so that an incoming optical signal from one of the optical paths 20-26 will impinge upon the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR). When a trench is filled with a liquid having a refractive index generally matching that of the waveguides, optical signals propagating along the lateral optical path that extends across that trench will be transmitted through that trench. However, when a bubble is formed within the trench, the optical signals are reflected by the wall of the trench from the lateral optical path to a vertical optical path that intersects the lateral optical path at the location of the trench.
The heater chip 14 of the optical switch 10 includes heating elements 38, i.e., resistors, and other electrical elements, such as transistors, to address individual resistors. For simplification, only the resistors are shown in FIG. 2. The heater chip is aligned with the waveguide chip 12 so that each resistor of the heater chip is positioned below a trench 36 of the waveguide chip, where two optical paths intersect. The resistors provide the thermal energy to create the bubbles within the trenches. Therefore, by selectively activating the resistors, any optical signals that were originally propagating through the lateral optical paths 20-26 can be rerouted to the vertical optical paths 28-34. The heater chip is attached to the metal substrate 16 of the optical switch, as shown in FIG. 1. The metal substrate contains a reservoir 40 of the refractive index-matching liquid. The reservoir is connected to the trenches of the waveguide chip by vias (not shown), which extend through the heater chip.
In order to provide an optimized and consistent performance of the thermally activated optical switch 10, the heater chip 14 needs to be maintained at nearly constant and uniform temperature. Large temperature variations at the intersections of optical paths, or cross points, where the bubbles are created to reflect optical signals, cause increased optical losses and cross talk, as well as perturbations to the bubble behavior. Environmental changes aside, accurate and precise temperature control of an NxN thermally activated optical switch, where N is significantly large, is difficult because N resistors can be activated simultaneously. Therefore, the total heat load of the heater chip can change by as much as N times the power required at each cross point, which results in large temperature variations.
An active temperature control device 42, shown in FIG. 1, can be utilized to try to control the temperature fluctuations within the optical switch 10. However, for packaging reasons, the temperature control device is located at a significant distance from the heater chip 14, where the sudden heat load changes are generated. As shown in FIG. 1, the temperature control device is located below the optical switch. Thus, the temperature control device and the heater chip are separated by the metal substrate 16. This implies that (i) a thermal gradient will exist between the heater chip and the temperature control device and (ii) any change in the local temperature of the switch will be resolved on a time scale limited by the heat conduction between the heat generating resistors and the active temperature control device. The amplitude of the temperature fluctuations depends both on the power required for each resistor and on the thermal resistance of the path between the heat generating resistors and the temperature control device. Therefore, if the materials along the heat path have a low thermal diffusivity, the temperature control device will have a slow response time. It follows that on-chip temperature control will benefit from reducing the power requirements of the resistors and from devising packaging solutions that maximize the heat transfer between regions of heat production and heat removal.
Although the above approaches will result in an improved on-chip temperature control for a thermally activated optical switch, additional improvement in temperature control is desired. Therefore, what is needed is a thermally activated optical switching device and a method for improving the temperature control of the switching device.